This provides access to the calling context of SOAP::Deserializer.
In a client side context the often means a reference to an instance of SOAP::Lite.
In a server side context this means a reference to a SOAP::Server instance.

A useful utility for SOAP::Deserializer is for parsing raw XML documents or fragments into a SOAP::SOM object.
SOAP::Lite developers use this technique to write unit tests for the SOAP::Lite module itself.
It is a lot more efficient for testing aspects of the toolkit than generating client calls over the network.
This is a perfect way for developers to write unit tests for their custom data types for example.

Here is an example of how raw XML content can be parsed into a SOAP::SOM object by using SOAP::Deserializer: